A large number of possibilities are known for detecting objects in a region or zone to be examined. One possibility for detecting objects is the use of light barriers. The term light barrier is understood hereinafter likewise to include the terms proximity sensor and motion sensor. A light barrier is an electronic-optical system comprising at least one emitter and at least one detector. A semiconductor chip which emits an electromagnetic radiation is considered to be an emitter. The emitter is also referred to as transmitter or radiation source. The detector, by contrast, is a semiconductor chip which detects an electromagnetic radiation, and which is also referred to as sensor or receiver.
The functional principle of a light barrier will be explained briefly below. The emitter emits an electromagnetic radiation in a specific wavelength range with a corresponding light intensity I. The detector is sensitive at least to this specific wavelength range of the radiation. The beam path between emitter and detector is altered by an object to be detected in the light barrier. The resultant alteration of the impinging luminance of the radiation is registered in the detector. This registration is converted into an electrical switching signal in a subsequent step, said signal then being processed further.
The luminance L is defined as the ratio of the emitted light intensity I to the size A of its apparent luminous area. Luminance has the SI unit candela cd per square meter m2. Luminance is the photometric equivalent to radiance L(λ) having the unit watt*meter−2*steradian−1 or W·m−2·sr−1 in radiometry.
In the case of light barriers, a distinction is made, in principle, between one-way light barriers and reflected light barriers. In the case of one-way light barriers, emitter and detector are opposite one another, wherein the main emission direction of the emitter lies directly in the direction of the detector. Accordingly, the detector constantly detects the luminance caused by the emitter. As soon as an object enters the beam path between emitter and detector, the beam path is interrupted. The resultant non-detection of the emitted radiation of the emitter is evaluated in a further-processing unit.
In the case of reflected light barriers, in contrast to the one-way light barrier, emitter and detector are not opposite one another and are furthermore preferably situated in a common housing. As a result, the construction is less complex. In principle, in the case of reflected light barriers, a distinction is made 30, between two methods for detecting objects.
In the case of the first method, a reflective element is arranged on a first side, and the housing with emitter and detector is arranged on an opposite side. The element and the housing are oriented in such a way that an interruption of the beam path by an object causes a change in luminance in the detector.
Alternatively, no reflector is used. If an object is in the beam path of the emitter, then the radiation of the emitter impinges on the object and is scattered at the object surface on account of the surface configuration of the object. As a result of the scattering, part of the emitted radiation will impinge on the detector. The change in the luminance in the detector is registered.
A general problem in the configuration of light barriers is the influence of environmental variables, for example daylight, extraneous light or the like.
Detector and emitter are increasingly being accommodated on a carrier within a housing. Electrical driving arrangements can thereby be configured in a simplified manner. As a result, it is necessary to take account of a further problem in the configuration of light barriers, so-called crosstalk between the emitting semiconductor chip and the detecting semiconductor chip. This crosstalk occurs if, for example, radiation is coupled into the detector on a direct path from the emitter before said radiation has left the housing. Crosstalk likewise occurs if the radiation is reflected within the housing and impinges on the detector. The radiation which reaches the detector as a result of crosstalk changes the operating point on the characteristic curve of the detector. The sensitivity of the detector is sometimes greatly impaired by the change in operating point. In order to increase the sensitivity of a light barrier, it should always be endeavored to minimize the environmental influences and the crosstalk.